Adds a new device interface to xend/xm similar to the one for ioports

Adds a new device interface to xend/xm similar to the one for ioports
where an interrupt (irq) can be specified in the configuration file
and permission to bind to that interrupt will be given to a driver
domain.

This functionality could be used when trying to give a legacy device
to a driver domain. For example, by specifying the i/o ports and the
interrupt of your serial controller, you can use this patch to put a
serial port in a driver domain. This was discussed briefly on
xen-users in regards to using an external modem in a domU:

http://article.gmane.org/gmane.comp.emulators.xen.devel/20573
http://article.gmane.org/gmane.comp.emulators.xen.user/8754

Also adds the ability to specify irqs on the xm command-line when
creating a domain (the following examples show a serial port being
given to a domU):

xm create serial-dd irq=4 ioports=03f8-03ff

or in a flat config-file:

irq = [4]
ioports = ['03f8-03ff']

or in an SXP config file:

(device (irq (irq 4)))
(device (ioports (from '0x03f8')(to '0x03ff')))

The ioports functionality is already in xen, but this patch also fixes a
bug with it that I assume cropped up due to changes in xend since it was
submitted ('dev' doesn't exist in iopif.py, returning 'None' seems to be
the solution).

Also adds some in-line documentation in tools/python/xen/xm/create.py
to be more accurate regarding adding pci devices and i/o ports on the
command-line.

Signed-off-by: Ryan Wilson <hap9@epoch.ncsc.mil>